Evidence is presented that pairing between the human X and Y chromosomes could be more extensive at early pachytene than has previously been supposed and could involve even the entire euchromatic portion of the Y chromosome. Following desynapsis over the major part of the X and Y axes, a small paired segment of Xp and Yp remains into late pachytene. Association between the distal tips of Xq and Yq can also be observed in about one half of the spermatocytes examined. A hypothesis linking meiotic pairing to early replicating sites along the chromosomes is proposed.
In situ hybridization to human meiotic metaphase I (MI) preparations, using the labeled minisatellite core sequence λ33.15, showed clustering of autoradiographic grains principally at or around chiasmata, autosomal sites where crossing-over had occurred. For the XY bivalent, the pairing region formed between the terminal regions of the two short arms (Xpter-Ypter), was also a principal site of labeling; in addition, the terminal region of the X long arm (Xqter) was labeled. Control experiments using a member of the human Alu family of dispersed repeated DNA sequences showed a much more randomized grain distribution, with clustering over chiasmata being far less obvious. The data provide support for the suggestion that polymorphic minisatellite regions within the human genome might play a significant role in pairing and/or recombination.
Meiotic studies have been carried out in five human male translocation heterozygotes. An account of the cytological observations is given together with information concerning their reproductive history and fertility.
Two cases of human reciprocal X-autosome translocation, t(X;12) and t(X;2), are described in sterile males, along with meiotic findings. Each carrier had inherited the translocation from his mother. Both showed azoospermia and germ-cell maturation arrest at the primary spermatocyte level, with most cells being arrested at the pachytene stage. A few metaphase I (MI) divisions were found, with occasional metaphase II cells being seen in the t(X;2) carrier. MI air-dried preparations gave clear evidence of chain quadrivalent formation. In the t(X;2) heterozygote, the pairing characteristics of the quadrivalent at pachytene were also analyzed in electron microscopic spreads. Disturbance of pairing around the breakpoints characterized most quadrivalents, and there was evidence in about 20% of the cells that nonhomologous pairing had taken place between the translocated chromosomes and the normal chromosome 2. Comparisons are made with similar nonhomologous pairing configurations seen at pachytene in quadrivalents of male reciprocal X-autosome translocations of the mouse.
Anecdotal reports of fertility in female mules (jack donkey × mare) and hinnies (stallion × jenny donkey) have appeared in the literature over the years, but scientists have generally regarded them with scepticism. The fact that some of these hybrids can come into estrous and ovulate makes fertility conceivable, given that opportunity for mating arises. In China, where mules are bred extensively for work on the farms, a fertile female mule and a fertile female hinny have now been verified by chromosomal investigation. Each had mated with a donkey and produced a filly foal. The foals show unique hybrid karyotypes different from the mule’s or hinny’s and different from each other’s. The studies make it clear that mule and hinny fertility, at least for the female hybrid, is a real possibility.
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